Electrical contact brush



Patented Sept. 7, 194a ELECTRICAL CONTACT BRUSH Dimiter Ramadanofl,Cleveland, Ohio, assignor to National Carbon Company, Inc., acorporation of New York No Drawing. Application April 29, 1944,

' Serial No. 533,401

(Cl. 17l325) 2 Claims.

The invention relates to electrical contact brushes, and morespecifically to carbon brushes for use in electrical machinery requiredto operate under conditions such as exist in the atmosphere at highaltitudes.

Well-made brushes composed of natural or artiiicial graphite, or othercommercial forms of carbon, or of mixtures of carbon and one or more 1wheel, and instances are known where brushes" have worn out completelyin less than an hour at these altitudes. The most suitable of the knownbrush compositions may in some instances last several hours; but at bestthe brushes can not be relied upon for long.

Modern aircraft use electrical motors, generators, and other electricaldevices requiring contact brushes. Both military and civil aircraft arebeing designed to operate at great altitudes, and there is an urgentdemand for electrical contact brushes that will operate dependably overa useful long life at such altitudes. The principal ob- J'ect of thisinvention is to meet this demand.

The reasons for the poor behavior of brushes at high altitudes are notaltogether clear; but it seems certain that two important factors arethe dryness and the low oxygen pressure of the air. A similar behaviorhas sometimes been observed, during periods of unusually dry winterweather, even at low altitudes in the cases of some brush compositions;but the problem has not been a serious one. By operating brushes in atest chamber in which the atmosphere can be controlled, it has beenfound that either a low moisture content or a low partial pressure ofoxygen may 7 z Further support for this hypothesis, and a sug gestionthat other film-forming materials might be substituted for moisture andoxygen, are found in the observation that new electrical apparatus,still containing traces of slightly volatile compounds in its insulationand paint, may not give trouble at high altitudes until after severalweeks or months of use. This latter circumstance increases thehazard,because rapid brush wear may start without warning after a considerableperiod of satisfactory operation. In an airplane several hours from itsbase the brushes may suddenly start to wear at a rate which will destroythem in an hour.

One approach to the problem is to incorporate in the brush, orimpregnate the brushes with, one or more substances which will establishand maintain a suitable film between the commutator or slip-ring andthecarbon brush. The choice of a suitable material is a difficult one, formuch more is involved than the mere provision of a film-formingmaterial. The material must be effective in'small amounts, because theinterstitial storage space in a brush body is not very great. If it isto bean impregnant, it must be a liquid or liqueflable by melting or bysolution to make impregnation possible, and it must have ability topenetrate thoroughly both inner and outer pores of the carbon. In anycase, it must not evaporate rapidly, and it must not exude, forexudation may result not only in undue depletion of the impregnant butalso in the formation of a surface layer which tends to pick up dirt andto interfere with the smooth operation of the brush in its holder.Finally, the film which the material supplies must have a low electricalresistance, must not pick up dirt nor decompose to deleterious products,and must be stable under a wide variety of conditions of atmosphere,temperature, and of mechanical and electrical stresses. Dozens of brushimpregnants, including I such materials as paraffin wax, beeswax,carnaucause rapid brush wear, and it has also been the conditionsexisting at altitudes above 25,000

feet, and at the same time satisfied the-other requirements indicated.

'The principal object achieved by this invention is a carbon brushhaving improved life and good operating characteristics at altitudeswell above 25,000 feet, for instance at 40,000 feet.

The invention is an electrical contact brush comprising carbon orgraphite, which may inments thorium, cerium, and yttrium. Sometimes thecommercial mixtures are byproducts of the manufacture of thorium orcerium and therefore contain but little of these elements. of any'ofthese mixtures, as well as of the commercially pure thorium, cerium, oryttrium, may be used in this invention. The particular compound used isnot of controlling importance, but the ready availability of the oxidesand salts particularly recommends their use. Fluorides are quitesatisfactory.

The weight percentage of rare earth element compounds is not critical,but in general it is prei'erred that as much be added as possiblewithout materially decreasing the strength and electrical conductivityof the brush. Rare earths or rare earth metal fluorides amounting toabout 15% by weight of a graphite-copper brush is suggested as asuitable, but not limiting, proportion.

Insoluble compounds, such as the oxides, or

compounds of low solubility in common solvents,

may be added as dry powder to a powdered graphite, carbon, orgraphite-metal molding mixture which is then formed, compacted, and heattreated in conventional manner. Soluble compounds,

Compounds such as the chlorides or acetates, are preferably incorporatedby impregnating a completed brush with a solution of the compound andremoving the solvent by evaporation. The impregnation can mostconveniently be effected under vacuum. Such soluble compounds may thenbe converted to insoluble compounds, for example by heating or bytreating with a suitable reagent such as an acid.

Brushes embodying this invention wear at only a very slow rate (in aparticular typical instance 0.01 inch per hour on a brush containingabout 15% rare earth oxides or fluorides) at the low pressuresencountered at very high altitudes above 25,000 feet. Moreover, they arecharacterized by low friction, and low total contact voltage drop, lnsliding contact with copper.

I claim:

Electrical contact brush comprising a major proportion of a mixture ofgraphite and copper, and a minor proportion of at least one of the rareearths of the thorium, cerium, and yttrium groups, such rare earth orearths amounting'to about 15% by weight of the brush.

2. Electrical contact brush comprising a major proportion of a mixtureof graphite and copper, and a minor proportion of at least one fluorideselected from the fluorides of the thorium, cerium, and yttrium groupsof rare earth elements, such fluoride or fluorides amounting to about15% by weight of the brush. 1

DIMITER RAMADANOFF.

' REFERENCES CITED The following references are of record in the filo ofthis patent:

UNITED STATES PATENTS Number v Name Date 1,039,522 Guay Sept. 24, 19121,147,422 Scott July 20, 1915 1,173,370 Molt Feb. 29, 1916 1,867,524Orne July 12, 1932

